Thick film electroluminescent ("EL") lamps are well known and generally comprise a phosphor between an optically transparent front electrode layer and a back electrode layer, all covered by a protective layer. The two electrodes are generally planar layers, but may be grids of electrically conductive material disposed at right angles to each other so that the phosphor at selected grid coordinates can be excited.
In general, different methods are used for depositing the various layers of thin film lamps than are used for thick film lamps. In contrast to thin film lamps made by vacuum deposition of the various layers, usually on glass, thick film lamps are generally made by roller coating the various layers, i.e., from the back forward on foil or a metalized polyester back electrode, or from the front backward on an indium tin oxide ("ITO") sputtered heat stabilized polyester used as the transparent front layer. More recently as shown for example in the Mental U.S. Pat. No. 4,626,742 dated Dec. 2, 1986, the various layers including the electrical connections thereto have been screen printed on a transparent polyester base, particularly where the lamps are of unusual sizes and/or shapes.
A problem common to all of the known techniques for making lamps is the protection of the layers, particularly the phosphor, from moisture. Moisture gives rise to dielectric breakdown and is highly detrimental to both lamp longevity and performance.
Moisture is particularly a problem in the favored screen printing process where the binder resin for the various layers is generally hygroscopic, and where the highly hygroscopic cyanoethly cellulose, often blended with cyanoethyl starch or sucrose, are commonly used. The traditional approach to the moisture problem, as shown for example in the Schimizu U.S. Pat. No. 5,188,901 dated Feb. 23, 1993 and the Kawachi U.S. Pat. No. 4,767,679 dated Aug. 30, 1988, is the encapsulation of the entire lamp in a fluoropolymer or PTCFE film. However, such encapsulation is an expensive and time consuming process.
It is accordingly an object of the present invention to obviate the moisture problem by use of a non-hygroscopic binders throughout the manufacturing process.
It is another object of the present invention to provide a novel EL lamp and process in which the need for an external protective encapsulation is obviated.
One known method of attacking the moisture problem has been to coat the individual phosphor particles with a thin layer (e.g., 0.4 microns) with aluminum oxide. Such lamps, when used with a traditional binder system and overcoating, have shown enhanced moisture resistance, but provide only about one half the illumination of the more traditional uncoated phosphor lamps for the electrical power applied to them.
It is accordingly an object of the present invention to reduce the moisture problem without sacrificing significant illumination by the use of individually coated phosphor particles.
Alternatively, it is another object of the present invention to use the moisture resistant characteristics of individually coated phosphor particles in a lamp capable of operating at higher voltage and frequencies so as to retain the illumination levels of non-coated phosphor particles.
In another aspect, the light emitted by a thick film EL lamp is of course related to the excitation of the phosphor by the electrical current through the lamp, and the current is inversely related to the capacitive reactance and is thus a function of the frequency of the applied electrical signal, i.e., the higher the frequency the lower the capacitive reactance and the brighter the lamp.
However, the use of high frequency excitation of EL lamps presents a problem in the stability of the dielectric. When a lamp is operated at high voltage and above 900 to 1,000 Hz, local heating in the dielectric layer is due to the resistive dissipation of heat in the dielectric/phosphor junction. For this reason, and because of brightness and color rendition, the specifications for most lamps require operation between 400 Hz and 2,000 Hz at 115 volts.
The failure of foil backed lamps from dielectric breakdown is catastrophic, for a low impedance shunt is thereby established between the electrodes. For ITO sputtered polyester front electrode lamps, the electrode generally fuses to open the circuit around the area of dielectric breakdown, producing a dark spot. As the dielectric continues to breakdown, other spots appear quickly degrading the performance of the lamp to an unacceptable level.
It is accordingly an object of the present invention to reduce the incidence of the dielectric breakdown in EL lamps by the use of a common binder in the phosphor and dielectric layers.
It is another object of the present invention to provide a novel EL lamp and process in which the diffusion at the dielectric/phosphor junction is significantly increased.
It is still another object of the present invention to provide a novel EL lamp and process in which the effective surface layer of the phosphor is significantly increased.
It is yet another object of the present invention to provide a novel EL lamp and process which is capable of both continued operation at a higher excitation frequency and intermittent operation (and thus high brightness) at greatly increased excitation frequencies.
In still another aspect, the temperature at which an EL lamp operates often causes delamination or separation of the various layers because of unequal coefficients of expansion of the binders used in the layers. Uneven expansion causes flexing and localized stress which often increases the incidence of failure.
It is accordingly an object of the present invention to obviate the problem of localized stress by use of binders for the various layers which have an uniform coefficient of thermal expansion.
It is another object of the present invention to provide a novel EL lamp and process with significantly reduced mechanical damage as a result of the inherent localized thermal effects due to lamp operation.
In yet another aspect, the darkening of the phosphor in thick film EL lamps has been a problem as a result of ultraviolet ("UV") radiation from exposure to sunlight. The prior art has attempted to reduce the darkening of the phosphor from exposure to sunlight by laminating or coating the lamp with a UV resistant layer, but such layers and coatings have proven expensive and time consuming in the manufacturing process.
It is accordingly an object of the present invention to reduce the problem of UV degradation of the phosphor in an EL lamp by the use of a binder for the phosphor which is UV resistant.
It is another object of the present invention to provide a novel EL lamp and process with significantly reduced UV susceptibility without the need for an additional UV resistant layer or coating.
The control of different areas of illumination has long been a problem, and is addressed in the present invention by the use of multilayers, i.e., different phosphors may be screen printed or otherwise layered on the lamp in different steps to produce different colors, and the layering of dielectrics and metalized conductors over previous conductive layers permits electrical access to, and thus electrical control over, various areas of the lamp.
It is accordingly an object of the present invention to provide a novel EL lamp and process with enhanced electrical control over various areas of the lamp by the layering of dielectrics and conductors, and to provide lamps with different colors in different areas.
These and many other objects and advantages will be readily apparent to one skilled in the art to which the invention pertains from the claims and from a perusal of the following detailed description of preferred embodiments when read in conjunction with the appended drawings.